1. Field of the Invention
The present invention relates to a method of converting colorimetric values measured by a first colorimeter into colorimetric values measured by a second colorimeter.
The present invention also relates to a color image output apparatus which is capable of correcting, at desired colorimetric values, colorimetric values measured by a built-in colorimeter which is incorporated in a color image output apparatus or colorimetric values measured by an external colorimeter connected on-line to the color image output apparatus to correct colors in three hues C, M, Y that are outputted by the color image output apparatus based on image data in three hues R, G, B inputted to the color image output apparatus.
2. Description of the Related Art
There are known color image output apparatus for producing color images by developing colors in given gradations with colorants in three hues C, M, Y. FIG. 10 of the accompanying drawings schematically shows such a color image output apparatus 1. In the color image output apparatus 1, as shown in FIG. 10, inputted image data in three hues R, G, B, i.e., device-dependent image data R, G, B, are converted in gradation by a lookup table (LUT) unit 5 having gradation-correcting one-dimensional LUTs 2, 3, 4, and the gradation-converted image data R, G, B are supplied to an exposure unit 6.
In the exposure unit 6, three laser diodes (not shown) are energized by the gradation-converted image data R, G, B from the LUT unit 5 to emit respective laser beams L in R, G, B. The laser beams L in R, G. B are applied to a film F to form a latent image on the film F. The latent image formed on the film F is then developed into a visible color image whose colors are expressed by three hues C, M, Y.
The color image output apparatus 1 is used as a proofer for a color printing press, for example. The color image output apparatuse 1 is used as a proofer because a color proof with a color image needs to be made for proofreading before an actual color print is produced by a color printing press such as a rotary press and also because such a proofer is capable of easily producing a plurality of color proofs, i.e., hard copies with color images thereon, in a short period of time as it does not require press plates which would be needed by color printing presses.
Therefore, the colors of a color print to be produced by a color printing press which will be used can be simulated with a color proof produced by the color image output apparatus 1, so that they can easily be confirmed prior to being actually printed.
The gradation-correcting one-dimensional LUTs 2, 3, 4, which are stored in a memory in the color image output apparatus 1, are prepared so as to meet standard printing conditions including inks, sheets, and printing press conditions. However, it is impossible for the gradation-correcting characteristics (also referred to as xe2x80x9cgradation characteristicsxe2x80x9d) of the one-dimensional LUTs 2, 3, 4 to be in full accord with the desired printing conditions of the printing press that is going to be actually used. For generating a color proof depending on the desired printing conditions, therefore, it is necessary to correct the gradation characteristics of the one-dimensional LUTs 2, 3. 4 depending on the desired printing conditions.
Specifically, if target gradations (target density gradations) for the hues C, M, Y on the film F with respect to the inputted image data R, G, B are set respectively to target gradations (target density gradations) Dc0, Dm0, Dy0 depending on the desired printing conditions, as shown in FIG. 11 of the accompanying drawings, then it is necessary to correct the standard gradation characteristics of the one-dimensional LUTs 2, 3, 4 which have been stored in advance to meet the standard printing conditions, so that the target gradations Dc0, Dm0, Dy0 will be achieved from the inputted image data R, G, B by the one-dimensional LUTs 2, 3, 4.
According to a gradation correcting process employed in the conventional color image output apparatus 1, as shown in FIG. 12 of the accompanying drawings, the image data R, G, B are incremented by respective given gradations, and then supplied through the standard one-dimensional LUTs 2, 3, 4 that have originally been incorporated in the color image output apparatus 1 to the exposure unit 6, which emit laser beams L in R, G, B to output monochromatic patches in the respective hues C, M, Y on the film F in step S1. Then, the patches in C, M, Y are measured for respective densities Dc, Dm, Dy thereof in step S2.
The measured densities Dc, Dm, Dy are then compared with the respective target gradations Dc0, Dm0, Dy0 shown in FIG. 11, and density differences therebetween are outputted in step S3. Thereafter, it is decided whether the density differences fall in a desired difference range or not in step S4.
Since the desired printing conditions are usually different from the standard printing conditions, the density differences usually do not fall in the desired difference range in step S4. The association (conversion relationship) between the image data R, G, B inputted to the one-dimensional LUTs 2, 3, 4 and the image data R, G, B outputted from the one-dimensional LUTs 2, 3, 4 is corrected on a trial-and-error basis depending on the density differences for thereby correcting the one-dimensional LUTs 2, 3, 4 in step S5. Hereinafter, the association (conversion relationship) means, e.g., both of the relationship converting the inputted image data into the outputted image data and relationship inversely converting the outputted image data into the inputted image data.
The loop of steps S1-S5 is repeated until the density differences fall in the desired difference range in step S4. When the density differences fall in the desired difference range in step S4, i.e., when the measured densities Dc, Dm, Dy differ from the respective target gradations Dc0, Dm0, Dy0 within the desired difference range, the one-dimensional LUTs 2, 3, 4 are properly corrected in gradation.
The above gradation correcting process employed in the conventional color image output apparatus 1, however, is time-consuming for the reason that since the gradation characteristics, which represent the association between the inputted and outputted image data, of the one-dimensional LUTs 2, 3, 4, are corrected on a trial-and-error basis, patches in C, M, Y need to be printed and measured for respective densities Dc, Dm, Dy each time the one-dimensional LUTs 2, 3, 4 are corrected depending on the density differences. In addition, the user of the color image output apparatus 1 has to be highly skilled in determining how much the one-dimensional LUTs 2, 3, 4 need to be corrected, i.e., corrective quantities for the one-dimensional LUTs 2, 3, 4, on the basis of the density differences.
The above problems may be solved by a process which will be described below. Inasmuch as the process is a novel process, it will be described briefly below and will subsequently be described in more detail with respect to an embodiment of the present invention.
According to this process, first, target gradations (referred to as xe2x80x9ctarget colorimetric valuesxe2x80x9d) are established such that a gray balance will be achieved by the color image output apparatus 1 when the image data R, G, B outputted by the color image output apparatus 1 are equal to each other, i.e., R=G=B (stated otherwise, when the inputted image data R, G, B pass through the one-dimensional LUTs 2, 3, 4 without being changed thereby i.e., the inputted and outputted image data of these LUTs 2, 3, 4 are in a 1:1 correspondence). Then, the color image output apparatus 1 outputs a color chart having color patches which represent equal image data R, G, B, respectively. The color patches are then colorimetrically measured, and a three-dimensional color-matching lookup table (hereinafter referred to as xe2x80x9cthree-dimensional CM LUTExe2x80x9d) for converting colorimetric values into R, G, B values is generated from the measured colorimetric values of the color patches. Then, using the one-dimensional LUTs 2, 3, 4, a gray balance is produced by the color image output apparatus 1 from the inputted image data R, G, B which are equal to each other, i.e., R=G=B, and colorimetrically measured to produce colorimetric values (referred to as xe2x80x9cmeasured colorimetric valuesxe2x80x9d). R, G, B values corresponding to the measured colorimetric values and the target colorimetric values are determined by way of volume interpolation by referring to the three-dimensional CM LUT. The differences between the R, G, B values thus determined are added respectively to the one-dimensional LUTS 2, 3, 4 for thereby correcting the one-dimensional LUTs 2, 3, 4.
Since the above process can determine automatically, not on a trial-and-error basis, corrective quantities for the one-dimensional LUTs 2, 3, 4, the one-dimensional LUTs 2, 3, 4 can be corrected accurately in a short period of time normally by one cycle of operation.
According to this process, however, the measured colorimetric values may be produced by a colorimeter (hereinafter referred to as xe2x80x9cfirst colorimeterxe2x80x9d to be corrected) incorporated in the color image output apparatus 1, and another colorimeter (hereinafter referred to as xe2x80x9csecond colorimeterxe2x80x9d serving as a reference colorimeter) may be used to generate the three-dimensional CM LUT.
The reasons for using such two colorimeters are as follows: For generating the three-dimensional CM LUT, since a great number of color patches need to be measured, a relatively large and expensive colorimeter capable of two-dimensional measurement (surface measurement) is used by a manufacturer of the color image output apparatus 1, and for correcting the one-dimensional LUTs 2, 3, 4, a relatively small and inexpensive colorimeter capable of spot measurement (point measurement) is used as a built-in colorimeter in the color image output apparatus 1. The color image output apparatus 1 which incorporates the first colorimeter has a memory which stores the three-dimensional CM LUT that has been generated using the second colorimeter. The user of the color image output apparatus 1 operates the first colorimeter, i.e., the built-in colorimeter in the color image output apparatus 1, to correct the one-dimensional LUTs 2, 3, 4.
Generally, colorimetric values measured of one gray chart vary from colorimeter to colorimeter. Even though the one-dimensional LUTs 2, 3, 4 are corrected in order to convert output image data thereof into the target R, G, B values, using the three-dimensional CM LUT, the colorimetric values produced by the first and second colorimeters remain different from each other, i.e., an error due to the different types of the colorimeters remains unremoved, resulting in a failure to achieve a full color match. Therefore, the above novel process still requires the one-dimensional LUTs 2, 3, 4 to be corrected by fine adjustment to intentionally shift the gradation characteristics representative of the association between the image data inputted to and outputted from the one-dimensional LUTs 2, 3, 4 after one cycle of correcting operation, for thereby enabling the color image output apparatus 1 to output desired colors.
It is therefore an object of the present invention to provide a method of converting colorimetric values measured by a first colorimeter into colorimetric values measured by a second colorimeter in order to, for example, be able to correct one-dimensional LUTs for respective three hues, which serve to make gradation adjustments to enable the color image output apparatus to output desired colors, within a short period of time without the need for correction by way of fine adjustment, even if a colorimeter incorporated in the color image output apparatus or a first colorimeter provided outside and connected on-line to the color image output apparatus and a colorimeter used to generate a three-dimensional CM LUT are different from each other.
Still another object of the present invention is to provide a color image output apparatus which is capable of correcting, at desired colorimetric values, colorimetric values measured by a built-in colorimeter which is incorporated in a color image output apparatus or colorimetric values measured by an external colorimeter connected on-line to the color image output apparatus to correct colors in three hues C (Cyan), M (Magenta), Y (Yellow) that are outputted by the color image output apparatus based on image data in hues R, G, B inputted to the color image output apparatus.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.